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FIELDIANA
Geology
Published by Field Museum of Natural History
Volume 33, No. 18 March 31, 1976
This volume is dedicated to Dr. Rainer Zangerl
The Brain of Mesonyx,
A Middle Eocene Mesonychid Condylarth
LEONARD RADINSKY
Department Of Anatomy
University Of Chicago
INTRODUCTION fHS y£**SC QE IHf
The Mesonychidae is a family of medium-sized to gigantic; am^^p^s
and carnivores that existed during the Paleocene and Eocene epochs in North
America, Europe, and Asia. Mesonychids were foiTnerljjpteffii^llVvlQ^lbttWOJS
donts as archaic members of the Order Carnivora (Simp^MRr^J?*Sfia most
earlier workers), but recently have been reassigned to the Order Condylar-
thra, to better reflect phylogenetic relationships (Van Valen, 1966; Romer,
1966). Condylarths were a heterogeneous group of early Tertiary, predomi-
nantly small to medium-sized omnivores and herbivores, from which the
various ungulate and subungulate orders were derived. For an introduction
to the literature on mesonychids, see Szalay and Gould (1966) and Szalay
(1969).
Endocranial casts of representatives of most of the families of condy-
larths have been described. These are of the arctocyonids Arctocyonides and
Arctocyon (Russell and Sigogneau, 1965); periptychid Periptychus (Tilney,
1931; Edinger, 1956); hyopsodontid Hyopsodus (Gazin, 1968); phenacodon-
tid Phenacodus (Tilney, 1931; Simpson, 1933); meniscotheriids Pleuraspido-
therium (Russell and Sigogneau, 1965) and Meniscotherium (Gazin, 1965);
and the tillodontid Tillodon (Gazin, 1953). Scott (1888) described a partly
exposed natural endocast of Mesonyx but his few observations, unsupported
by figures or measurements, provide no useful information. The endocast of
Mesonyx described below is important because it provides the first good
Library of Congress Card Number: 75-27501
Publication 1226 323
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5 cm
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Fig. 1. Mesonyx obtusidens, Yale Peabody Mus. 13141. Above, dorsal and lateral views
of endocast, approximately natural size. Below, lateral view of endocast in position in skull,
approximately x 2/5. Dashed lines indicate estimated boundaries of missing portions; dotted
iines indicate border of unexposed portions of endocast.
324
RADINSKY: BRAIN OF MESONYX 325
record of a mesonychid brain, and because it is one of the largest and one
of the latest condylarth endocasts known.
DESCRIPTION
A natural endocast was exposed by removing most of the right side of
the braincase of Yale Peabody Mus. 13141, a well-preserved, uncrushed skull
of Mesonyx obtusidens, from the Middle Eocene (Bridger B, about 50 million
years old), of Wyoming. The exposed portion of the endocast (fig. 1) includes
most of the cerebellum, cerebrum, and olfactory bulbs.
The rhinal fissure is located about two-thirds of the way down the
cerebrum, as seen in lateral view. It is well-marked caudally, but rostrally
is faint. A cast of a vascular sinus overlies its middle portion, a condition
commonly seen in other mammals. Three longitudinally oriented sulci divide
the neocortex above the rhinal fissure. The most lateral of these sulci is
unusually long and straight compared to what is seen in other mammals,
extending for most of the length of the hemisphere. The middle sulcus is
shorter, with a faint suggestion of a bifurcation at its rostral end. The medial
sulci is almost as long as the bottom one, and it appears to curve slightly
laterally at its rostral end.
Enough is preserved of the olfactory chamber to indicate that the olfac-
tory bulbs were slightly pedunculate and relatively small. The pyriform lobe
also appears to have been relatively small compared to the rest of the brain
(see fig. 2 for comparison with early mammals).
The midbrain was not completely overlapped, for there is a gap of about
5 mm. between the caudal end of the cerebrum and the cerebellar vermis.
No details of midbrain morphology are preserved in that space. The vermis
is clearly demarcated from the lateral hemispheres, but otherwise little sur-
face detail of cerebellar morphology is evident. The vermis is relatively high
and short, with a transverse groove located relatively rostrally. That groove
may represent the fissura prima, for on most mammal endocasts the / prima
is the most prominent, and often the only cerebellar fissure reproduced.
There is a faint indication of a longitudinally oriented goove on the side of
the cerebellar hemisphere; it may represent the boundary between the an-
siform lobule and the paraflocculus. The cerebellar hemispheres extend out
about as far laterally as the cerebral hemispheres.
From water displacement of a cast of the Mesonyx endocast, with olfac-
tory bulbs and the covered portion of the hind brain restored, I estimate the
endocranial volume to have been about 80 cc.
326 FIELDIANA: GEOLOGY, VOLUME 33
I have exposed the cerebrum of the Mesonyx endocast described by Scott
(1888), Princeton Univ. 10308. It is somewhat crushed and incomplete, but
appears similar in observable details to the Yale specimen. I see no basis for
Scott's description of the cerebral hemispheres as very small and the cerebel-
lum as relatively large.
MORPHOLOGICAL COMPARISONS
Factors to consider in comparisons with brains of other mammals are
body size, phylogenetic relationship, temporal relationship (geological age),
and ecological niche. Most closely related phylogentically to Mesonyx are
representatives of the other condylarth families. Of these, Arctocyonides,
Hyopsodus, Pleuraspidotherium, and Meniscotheriwn were considerably
smaller than Mesonyx. Therefore, the fact that their cerebral hemispheres
lacked convolutions or at most had a single neocortical sulcus (see references
cited above) does not necessarily indicate a less advanced stage of cortical
evolution than in Mesonyx, since degree of cortical folding in some groups
of mammals appears to be at least in part correlated with absolute brain size,
which, in turn, is correlated with body size. The influence of size on degree
of gyrencephaly can be seen in series of brains of living prosimian primates
(Radinsky, 1974) and ceboid primates (Hershkovitz, 1970). However, such
influence does not appear as evident in cercopithecid primates (Connolly,
1950; Radinsky, pers. observation) or in canid carnivorans (Radinsky, 1973).
Of the remaining condylarths for which endocasts are known, Peripty-
chus, Arctocyon, Phenacodus, and Tillodon were closer in body size to Meso-
nyx, although somewhat smaller. The endocast of Tillodon is crushed and
does not preserve enough surface detail for significant comparison with the
other genera. The brain of Periptychus, from the Middle Paleocene (about
65 million years old), is known from the dorsal half of an endocast of the
fore brain. It has a very high rhinal fissure, and only two small caps of
neocortex on top of the cerebrum. The brain of Arctocyon (fig. 2 A), from
Opposite:
Fig. 2. Drawings of endocasts of condylarths (A, B and C), an early ungulate (D), and
early carnivorans (E and F), in dorsal, lateral, and rostral views. See text for discussion.
Dashed lines indicate estimated boundaries of missing portions. A, Arctocyon primaevus,
redrawn from Russell and Sigogneau, 1965; B, Phenacodus primaevus, redrawn from Simpson,
1933; C, Mesonyx obtusidens, Yale Peabody Mus. 13141; D, Hyrachyus modestus, Amer. Mus.
Nat. Hist. 11713, with cerebellum restored from other specimens; E, Hyaenodon horridus,
Amer. Mus. Nat. Hist. 94760; F, Humbertia angustidens, Mus. Nat. Hist. Nat., Paris, from
a cast of the original specimen. Abbreviations: f, fissura prima; r, rhinal fissure. All drawings
to same scale, ?bout r. \/2.
A. Arc t oc yon
B. P h enacodus
C. M e s o n y x
D. H y r a c h y u s
F. H u m bertia
E H y a e n o d on
327
328 FIELDIANA: GEOLOGY, VOLUME 33
the Late Paleocene (about 60 million years old), was advanced over that of
Periptychus in having relatively more neocortex, evidenced by a slightly lower
rhinal fissure and the presence of one or possibly two neocortical sulci
(surface details are poorly defined on the two known Arctocyon endocasts).
The midbrain was widely exposed in Arctocyon. The brain of Phenacodus (fig.
2B), from the Early Eocene (about 55 million years old), was further ad-
vanced in having a lower rhinal fissure, with the neocortex covering more
of the midbrain and olfactory peduncles than in Arctocyon. One or possibly
two neocortical sulci were present in Phenacodus. (As in the cast of Arcto-
cyon, surface details are poorly preserved on the one described endocast of
Phenacodus.)
The brain of Mesonyx was advanced over those of the above mentioned
condylarths in having a relatively more expanded neocortex. This is indicated
by the relatively more ventrolateral position of the rhinal fissure, the presence
of three well-defined neocortical sulci, and the expansion of the cere -
brum above the height of the cerebellum. In addition, if the transverse
groove on the cerebellar vermis represents the Jlssura prima, it is in a more
rostral position than in the other condylarth endocasts, indicating expansion
of the neocerebellar portion of the vermis, a progressive trend presumably
correlated with the expansion of the neocortex of the cerebrum. The olfactory
bulbs are relatively smaller in Mesonyx than in the other condylarths in
which their size is known. Finally, even allowing for the more ventrally
located rhinal fissure, the pyriform lobe appears to have been relatively
smaller in Mesonyx than in the other condylarths.
There are no other condylarth endocasts of the same geological age or
younger than the Mesonyx endocast with which it may be compared. The
next most closely related group for which Middle Eocene endocasts are
known is the ungulate order Perissodactyla, which evolved from phenaco-
dontid condylarths in the Middle or Late Paleocene. Hyrachyus, a helaletid
tapiroid, was comparable in size to and contemporaneous with Mesonyx and
therefore suitable for comparison of external brain morphology. The brain
of Hyrachyus (fig. 2D) appears to have been similar in overall proportions
and degree of neocortical expansion to those of other Middle Eocene per-
issodactyls. Although the rhinal fissure was not as ventrally located in Hyra-
chyus as in Mesonyx, its brain also had three well developed neocortical sulci
and a fourth short one. However, the rostral end of the most lateral sulcus
curved medially and delimited a portion of frontal cortex in Hyrachyus that
is not so bounded in Mesonyx. The brain of Hyrachyus further differed from
that of Mesonyx in having less reduced olfactory bulbs and pyriform lobe,
a more caudally located fissura prima, and in being narrower across the
cerebellum than across the cerebrum. Thus in degree of neocortical expan-
RADINSKY: BRAIN OF MESONYX 329
sion, brains of Mesonyx and Hyrachyus appear to have been similar, although
there are differences in details (e.g., lower rhinal fissure in Mesonyx and more
differentiated frontal pole in Hyrachyus). The more rostrally located fissura
prima suggests that the cerebellum of Mesonyx was more advanced than that
of Hyrachyus. The relatively smaller olfactory bulbs and pyriform lobe of
Mesonyx are also specialized features.
While mesonychids are phylogenetically closer to condylarths and un-
gulates than to carnivorans, in general habitus they appear to have been more
similar to the latter, particularly during Eocene times (see Szalay and Gould,
1966). Therefore, it is of interest to compare the Mesonyx endocast with those
of early carnivorans. Archaic carnivorans, called creodonts, unrelated to the
ancestry of modern carnivorans, were abundant during the Eocene.
However, the earliest known creodont endocasts from animals close in size
to Mesonyx are from the Oligocene, about 15 million years later in time. The
brain of Hyaenodon horridus (fig. 2E), a hyaenodontid creodont, was more
advanced than the other known Eocene and Oligocene creodont brains. It
had two major neocortical sulci, and two shorter, variably developed ones.
The lower major sulcus curved medially at its rostral end, as in the Eocene
perissodactyls and unlike the straight lower sulcus in Mesonyx. However,
despite the expansion of the neocortex indicated by the presence of so many
sulci, the rhinal fissure in Hyaenodon was not as ventrally displaced as in
Mesonyx. Also, the olfactory bulbs and pyriform lobe are relatively larger
and the cerebellar fissura prima apparently less rostrally displaced in Hyaeno-
don than in Mesonyx.
The modern families of carnivorans, or neocarnivorans, appear to have
arisen from a late Eocene adaptive radiation of miacid carnivorans. Judging
from the various known Oligocene neocarnivoran endocasts (see Piveteau,
1951; Radinsky, 1971, 1973), the basal neocarnivoran brain was probably
similar to that of Humbertia angustidens (described under the name Viver-
ravusby Piveteau, 1962), a late Eocene miacid. The brain of Humbertia (fig.
2F) resembled that of Mesonyx in the position of the rhinal fissure, but
differed in having only two neocortical sulci (the coronolateral and suprasyl-
vian sulci), with a wide unfolded area of cortex between the lower sulcus and
the rhinal fissure. Unlike the conditions in Mesonyx, the sulci in Humbertia
are gently arched; in later carnivorans, the arching becomes even more
pronounced. Olfactory bulbs were relatively larger in Humbertia and the
fissura prima less rostrally displaced than in Mesonyx. Because of the differ-
ence in overall brain size, it is difficult to estimate the relative size of the
pyriform lobe in Humbertia compared to Mesonyx. The midbrain is com-
pletely covered by the cerebrum in Humbertia.
330 FIELDIANA: GEOLOGY, VOLUME 33
Because Humbertia was considerably smaller than Mesonyx, it would
be of interest to determine to what degree allometry was responsible for the
observed differences in brain morphology, particularly in the number of
neocortical sulci. Oligocene neocarnivorans that were closer in size to Meso-
nyx, such as the amphicyonids Amphicyon and Daphoenus, and the canid
Mesocyon, had brains that were similar in most features to that of Humbertia,
but had in addition a third sulcal arch, the ectosylvian sulcus, beneath a more
convex suprasylvian sulcus (Beaumont, 1964; Radinsky, 1971, 1973). Small
early neocarnivorans generally lack an ectosylvian sulcus, which suggests
that its absence in the Late Eocene Humbertia might be due to allometry.
In addition to the ectosylvian sulcus, some but not all large early neocarnivo-
rans have one or two short secondary sulci, the ectolateral and entolateral
sulci, adjacent to the caudal end of the cornolateral sulcus.
RELATIVE BRAIN SIZE
Most of the statements in the literature on relative brain size of fossil
mammals are unsupported assertions that brains were relatively small in any
given extinct species. For example, Scott (1888, p. 155) wrote that Mesonyx
had an exceedingly small brain capacity, but did not specify his point of
comparison, and gave no measurements of endocranial capacity. However,
during the past 10 years, Jerison (1973 and references cited therein) has
provided a large body of quantitative data on relative brain size in fossil
mammals, based on endocranial volumes (used interchangeably with brain
weights) and body weights (estimated from various skeletal measurements).
For purposes of comparison, Jerison uses an Encephalization Quotient, or
EQ, which is defined as the endocranial volume (or brain weight) of a given
species divided by the endocranial volume one would expect to find in an
"average" living mammal of that species' body weight. The relationship
between brain weight and body weight in Jerison's "average" living mammal
is described by the equation, E = 0.12 P0-67, (E = brain weight; P = body
weight), based on a large sample of living mammals. Bauchot and Stephan
(1966) compare relative brain sizes of recent mammals in a similar manner,
except their Encephalization Index is based on a comparison with the brain
size one would expect in a basal insectivore of a given body weight. For brain
weight-body weight comparisons, I would have preferred to use Bauchot and
Stephan's basal insectivore line as a standard, since the equation describing
relative brain size in basal insectivores is unlikely to change with the addition
of more data, while the equation for the "average" living mammal is more
likely to vary depending on what species are included in the sample.
However, since Jerison has calculated EQs for a large number of fossil
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332 FIELDIANA: GEOLOGY, VOLUME 33
mammals, to facilitate comparisons with his data I have used EQ to provide
a quantitative measure of relative brain size in Mesonyx.
Data for estimations of relative brain size in Mesonyx and other condy-
larths for which such data are available are presented in Table 1 . Endocranial
volumes were measured by water displacement of endocasts, (or copies of
endocasts), with distorted or missing parts restored. My estimate of endo-
cranial volume Arctocyonides differs from that of Jerison (1973, Table 11.1).
Body weight was estimated from body length, using the equation P=.025 L3,
which was calculated by Jerison (1973, p. 53) from a large sample of living
ungulates and carnivores. For Arctocyon and Arctocyonides, Jerison used the
equation P = .050 L3 to calculate body weight, which resulted in lower EQs
than I calculated, but from the known skeletal remains of those genera I see
no reason not to use the same equation as for the other condylarths. Body
length (skull and trunk length) could be measured directly only in Phenaco-
dus; for the other genera I estimated it from the proportions of skull length
to body length in other specimens or in related genera.
The Encephalization Quotient for Mesonyx obtusidens is 0.46 or 0.57,
depending on the estimate of body length used. EQs for the other condylarths
ranged from 0.20 to 0.32, all under the Mesonyx minimum estimate. For
Middle Eocene perissodactyls, Jerison (ibid.) estimated three EQs, ranging
from 0.37 - 0.49; for the Hyrachyus specimen shown in Figure 2D, I estimated
an EQ of 0.36 or 0.52, depending on the body length estimate used. For four
Eocene and Oligocene creodont carnivores, Jerison (ibid.) calculated EQs
ranging from 0.33 - 0.55; for my Hyaenodon horridus specimen, I estimate
an EQ of 0.61. For a sample of early neocarnivorans, Jerision calculated EQs
ranging from 0.32-0.79. Thus, compared to contemporaneous perissodactyls
and to creodonts and early neocarnivorans, Mesonyx did not have a relatively
small brain.
For comparison of relative brain size of Mesonyx with modern species,
I calculated mean EQs and the observed range of EQs for representative
samples of living insectivorans (data from Bauchot and Stephan, 1966), of
artiodactyls (the dominant surviving ungulates), and of carnivorans (see table
2). Relative brain size of Mesonyx was in the upper part of the observed
range of relative brain size of living insectivorans, in the lower part of the
observed range of living artiodactyls, and around the lower end of the ob-
served range of living carnivorans.
One of the problems with analyzing relative brain size by the above
method is the uncertainty involved in estimating body weight of extinct
species from skeletal measures. Even where complete skeletons are available,
and body length can be measured directly, it is evident from the graph
presented by Jerison (1973, p. 53, fig. 2.9) that there is a high degree of
RADINSKY: BRAIN OF MESONYX 333
variability in the body length-body weight relationship. Unfortunately, Jeri-
son does not provide calculations of the variance, but it appears from his
graph that for a mammal of 100 cm. body length, the observed range of body
weight is from 10 kg. to about 35 kg. This problem might be minimized if
one could calculate EQs for several related, approximately contemporaneous
species, for errors of body weight estimates would probably be random and
therefore with a large enough sample would cancel each other out. However,
Mesonyx is the only mesonychid for which brain size can be estimated, and
there are not even any other condylarths of Middle Eocene age available with
which it can be compared. Therefore, it is desirable to have another method
of estimating relative brain size to provide a check on the Encephalization
Quotient calculated for Mesonyx.
Plots of brain weight vs. foramen magnum area for six groups of living
mammals (insectivorans, rodents, prosimian primates, artiodactyls, carnivo-
rans, and monkeys) show approximately the same relative relationships as
do brain weight-body weight plots of those groups (Radinsky, 1967). For a
sample of 164 recent mammal species of those six groups, the coefficient of
correlation (r) between foramen magnum area and body weight is 0.98.
Removing the influence of brain weight, the partial correlation of foramen
magnum area and body weight is 0.65. Therefore, it seems reasonable to
Table 2. Relative brain size in some living mammals.
Encephalization Quotients1 EQA2
Order Mean Observed Range Mean Observed Range
0.41 0.20-0.68
1.08 0.69-1.52
Insectivora3
0.47
0.24-0.83
(N = 24)
Artiodactyla4
0.81
0.39-1.29
(N = 36)
Carnivora'
0.89
0.52-1.80
(N = 48)
1.37 0.88-2.62
1 EQ is the brain size of a given species divided by the brain size expected for
an "average" living mammal of that species' body weight. See text for
further information.
2 Encephalization quotient based on comparison with foramen magnum area
rather than body weight. See text for further information.
3 Brain weight and body weight data from Bauchot and Stephan, 1966.
4 Body weight data from Kruska, 1973, and Walker, 1964.
5 Body weight data from Walker, 1964.
334 FIELDIANA: GEOLOGY, VOLUME 33
examine the relationship between brain size and foramen magnum area in
Mesonyx as a check on the relative brain size as estimated from body weight.
To facilitate comparisons, I calculated the equivalent of the EQ for foramen
magnum data. The EQA of a given species is the observed brain size of that
species divided by the brain size one would expect in an "average" living
mammal of that species' foramen magnum area. In my sample of 164 species
of insectivorans, rodents, prosimians, articdactyls, carnivorans, and mon-
keys, the average brain weight-foramen magnum area relationship is ex-
pressed by the equation E = 22.4 A1-48, or log E = 1.35 + 1.48 log A (A =
foramen magnum area, cm2). The results of this approach are presented in
Tables 1 and 2.
Relative brain size in Mesonyx based on the foramen magnum area
comparison is higher than that of Arctocyonides, Meniscotherium, and
Phenacodus, the other condylarths for which the relevant data are available,
and comparable to that of early perissodactyls and carnivorans (Radinsky,
unpublished data). This confirms the analysis based on body weight com-
parisons. However, compared to the recent species, EQAs of Mesonyx and
the other condylarths are higher relative to their EQs. Thus on the basis of
foramen magnum area, relative brain size in Mesonyx is above the observed
range of insectivorans, just above the mean for artiodactyls, and well within
the lower part of the observed range for carnivorans. Two possibilities to
account for this difference are that we have overestimated body weights for
the extinct genera (and thus have EQs that are too low), or that the relation-
ship between foramen magnum area and body weight is different in the living
species compared to the fossil ones.
CONCLUSIONS
The brain of Mesonyx was relatively larger and more advanced in terms
of expansion of neocortex (and probably also neocerebellum) than the other
known condylarth brains with which it may be compared. The latter,
however, are from earlier time periods than Mesonyx. The brain of Mesonyx
was roughly comparable in relative size and in degree of neocortical expan-
sion compared to brains of contemporaneous perissodactyl ungulates and
slightly younger (geologically) carnivorans. The cerebellar fissura prima ap-
pears to be rostrally displaced in Mesonyx compared to early ungulates and
carnivorans, suggesting a relatively more expanded neocerebellum.
The brain of Mesonyx was more specialized than that of other condy-
larths and of early ungulates and carnivorans, in having relatively small
olfactory bulbs and apparently a relatively smaller pyriform lobe. The rela-
tive size of the pyriform lobe is difficult to estimate, and its apparent reduc-
RADINSKY: BRAIN OF MESONYX 335
tion in Mesonyx may in part be an illusion resulting from the relatively great
expansion of the neocortex. If the pyriform lobe was indeed relatively small
in Mesonyx, that may be correlated with the reduction of the olfactory bulbs,
since the pyriform lobe cortex is usually considered to be mainly involved
in olfactory function.
The rhinal fissure was more ventrally displaced in Mesonyx than in the
early ungulates and carnivorans that had a similar number of neocortical
sulci, suggesting either a greater degree of neocortical expansion in Mesonyx
or that its sulci were shallower.
The sulcal pattern of Mesonyx is so different from that of ungulates and
carnivorans (or of any other mammal), that I hesitate to attempt to identify
sulci and interpret functional areas of the cortex. The only functional inter-
pretation that is apparent from the known brain morphology of Mesonyx is
reduction in importance of olfaction, indicated by the apparently reduced
olfactory bulbs. I see no features of the brain of Mesonyx that suggest phylo-
genetic affinity to any other group of mammals.
ACKNOWLEDGEMENTS
For permission to prepare and describe the endocast from the Yale
Peabody Museum Mesonyx skull, I am grateful to Prof. E. Simons. For access
to other specimens utilized in this study, I thank D. Baird, Princeton Univer-
sity; C. Dechaseaux, Museum National d'Histoire Naturelle, Paris; C. L.
Gazin, U. S. National Museum; and M. McKenna and R. Tedford, the
American Museum of Natural History. This work was supported in part by
National Science Foundation Grant GB 31242.
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